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JOURNAL OF PLANKTON RESEARCH | VOLUME 18 | NUMBER 4 | PAGES 615-634 | 1996
© Oxford University Press
research-article |
A hypothesis on Mesozoic vicariance in Hydromedusae
Institute of Taxonomic Zoology (Zoological Museum), University of Amsterdam PO Box 94766, 1090 GT Amsterdam, The Netherlands
Received on September 23, 1994; accepted on December 4, 1995 In the Hydromedusae, many species show a tendency to be less restricted to a special distribution pattern compared to most other planktonic organisms. Latitudinal belt-shaped patterns in Hydromedusae are usually broader than the comparable distributions of molluscs or crustaceans. As a rule, the bathypelagic Hydromedusae are less restricted to specific depth layers than other taxa. Taxa of Hydromedusae showing distributions correlated with water masses are less restricted to water masses than taxa of other groups. Many types of distribution, like Equatorial and Central water patterns, commonly found in most planktonic groups, are absent in Hydromedusae. and many species show a strong tendency for southward dispersal. The present-day patterns of Narcomedusae and Trachymedusae seem to originate from an Antarctic fauna, the other subclasses of Hydromedusae show distribution patterns around the Indo-Malayan Archipelago. It is presumed that the Hydromedusae reflect a post-Cretaceous dispersal from two faunal centres. lndo-Malaya and Antarctica, that already became separ ated in Eocene times. The two Eocene faunal centres can be derived from one continuous area in which Hydromedusae survived the period of extinction at the end of the Cretaceous. Splitting of this centre probably forms the vicariant event responsible for the development of the Narcomedusae and Trachymedusae in the Antarctic waters, and of the Anthomedusae, Leptomedusae and Limnomedusae in the Indo-Malayan waters.